Clear channel access methods, apparatuses, media and signals

ABSTRACT

Methods, apparatuses, media and signals for providing clear channel access on a network are disclosed. A first method involves receiving a communication signal from a remote network element. The communication signal includes a previous transport overhead (PTOH) portion indicative of transport overhead contents of the communication signal prior to arrival at the remote network element, and a previous path error (PPE) portion indicative of path errors present in the communication signal at the remote network element. The method then involves modifying a transport overhead portion of the communication signal in response to the PTOH and PPE portions. A second method involves inserting into a communication signal received at a network element, a previous transport overhead (PTOH) portion indicative of transport overhead contents of the communication signal prior to arrival at the network element, and a previous path error (PPE) portion indicative of path errors present in the communication signal at the network element. The second method then involves transmitting the communication signal to a remote device.

FIELD OF THE INVENTION

The present invention relates to network communications, and moreparticularly, to methods, apparatuses, media and signals for providingclear channel access on a network.

BACKGROUND OF THE INVENTION

Carriers or operators of high bandwidth networks, such as opticalnetworks, typically sell network access to their customers, which ofteninclude telephone companies or other telecommunications serviceproviders. For example, an operator of an optical network segmentextending between two major cities may receive a number ofcommunications signals or channels from a number of respective customersat an originating city, which are then multiplexed or combined into asingle higher-speed optical communications signal. The higher-speedsignal is then relayed to the destination city, where it is thendemultiplexed or split apart into its component signals, which are thenseparately provided to the respective customers' facilities in thedestination city.

Each communications signal initially provided by a customer of thenetwork operator typically includes a payload portion in which theactual “live” communications traffic is stored, and further includes atransport overhead portion which is used by the customer for variouspurposes, including monitoring the occurrence of transmission errorsthat may arise on the customer's own communications equipment andfacilities.

However, as each customer's communications signal is carried over thenetwork operator's optical network segment extending between the twocities, it generally passes through a number of different networkelements at which it is necessary for the network operator's equipmentto over-write the customer's transport overhead data, in order for thenetwork operator to monitor the occurrence of errors on the opticalnetwork segment.

Accordingly, when the communications signal is relayed to the customerat the destination city, much of the customers transport overheadinformation has been destroyed. This may interfere with or destroy theability of the customer to monitor aspects of its own facilities, suchas the occurrence of errors on the customers equipment in the vicinityof the originating city, for example.

Accordingly, it would be desirable, from the point of view of some suchcustomers, for the network operator to be able to provide a “clearchannel” across the network segment, or in other words, for the networkoperator to pass transport overhead information to the customer at thedestination city in such a way that the customer's ability to monitorthe occurrence of errors on its own facilities would not be affected byanything that may have occurred over the operator's network segment, asif the network segment did not exist. One approach to a similar probleminvolves providing special dedicated facilities on the network segment,including a transparent multiplexer or combiner for example, along withspecial line facilities that preserve portions of the customer'sincoming overhead information for reconstruction at the destination, sothat the preserved overhead is effectively passed through the networksegment transparently. Disadvantageously, however, such dedicatedfacilities are not capable of combining a mixture of transparent andnon-transparent channels into a single optical signal, with the resultthat such facilities are useful only in circumstances where all of thecustomers whose signals are to be combined together desire transparentaccess to the network segment. In addition, if the individual signalsthat have been multiplexed into the single optical signal are not alldestined for the same network node or location, it is not possible forthese dedicated facilities to perform the usual seamless extraction ofthe individual signal at the intervening node at which the individualsignal is to be dropped off. Rather, the entire signal must bedemultiplexed in order to extract the individual signal that is to bedropped off, and the individual signals that are destined for subsequentnetwork locations must be re-multiplexed or combined back into a newoptical signal. This results in significantly increased equipment costs,as additional demultiplexers and re-multiplexers must be provided at anysuch intervening network node for signals that are merely passingthrough the network node. Accordingly, these dedicated facilities arenot well-suited to accommodating the differing needs of differentcustomers, and result in significantly increased equipment costs fornetwork operators.

Accordingly, there is a need for an improved way of providing clearchannel access.

SUMMARY OF THE INVENTION

Aspects of the present invention address the above needs by providing amethod and an apparatus for providing clear channel access on a network.The method involves receiving a communication signal from a remotenetwork element, the communication signal including a previous transportoverhead (PTOH) portion indicative of transport overhead contents of thecommunication signal prior to arrival at the remote network element, anda previous path error (PPE) portion indicative of path errors present inthe communication signal at the remote network element. The methodfurther involves modifying a transport overhead portion of thecommunication signal in response to the PTOH and PPE portions. Theapparatus includes a receiver operable to receive the communicationsignal, and further includes a processor circuit in communication withthe receiver and configured to modify the transport overhead portion.

Advantageously, by modifying the transport overhead portion of thecommunication signal in response to both the previous transport overheadportion and the previous path error portion, a clear channel may beprovided, allowing a customer who then receives the communication signalto process the transport overhead portion as if it had not been affectedby its passage across the network.

In addition, the above method and apparatus permit implementation innetwork configurations other than dedicated clear channel systems,thereby allowing the communication signal to pass through normal networkelements such as Line Terminating Equipment, Add/Drop Multiplexers, orRing configurations, for example. Thus, the disadvantages associatedwith dedicated clear channel facilities may be avoided, if desired. Forexample, if desired, the method or apparatus may be implemented intypical network configurations, allowing both clear channel andnon-clear channel communication signals to be multiplexed together, andallowing individual signals to be dropped off at intervening networknodes in the usual manner, without the need to demultiplex andre-multiplex the individual signals that are passing through to asubsequent network node.

The communication signal may include a plurality of component signals,in which case modifying the transport overhead portion preferablyinvolves calculating, for each of the component signals, a differencebetween path errors present in the component signal and path errorspresent in the component signal at the remote network element. Modifyingthe transport overhead portion may then include calculating a sum of thedifferences of each of the component signals, and adding the sum of thedifferences to at least some contents of the PTOH portion.Advantageously, these additional features further improve thetransparency of the clear access channel.

A further aspect of the invention provides a computer-readable mediumfor providing codes for directing a processor circuit to modify thetransport overhead portion in response to the PTOH and PPE portions.Similarly, another aspect of the invention provides a signal embodied ina carrier wave, the signal including code segments for directing aprocessor circuit to modify the transport overhead portion in responseto the PTOH and PPE portions. An additional aspect of the inventionrelates to an apparatus for providing clear channel access on a network,the apparatus including provisions for carrying out the above method.

In accordance with further aspects of the invention, there are provideda method and an apparatus for providing clear channel access on anetwork. The method involves inserting into a communication signalreceived at a network element, a previous transport overhead (PTOH)portion indicative of transport overhead contents of the communicationsignal prior to arrival at the network element, and a previous patherror (PPE) portion indicative of path errors present in thecommunication signal at the network element. The method further involvestransmitting the communication signal to a remote device. The apparatusincludes a processor circuit configured to insert the PTOH and PPEportions into the communication signal, and a transmitter incommunication with the processor circuit and operable to transmit thecommunication signal to a remote device.

Inserting the PTOH and PPE portions into the communication signal in theabove manner, and transmitting the signal to the remote device, allowsthe remote device to use the communication signal to provide clearchannel access, as described above, resulting in similar advantages tothose mentioned above.

If desired, inserting the PPE portion may involve performing TandemConnection Monitoring (TCM) or, advantageously, may involve a variationthereof. For example, if desired, rather than inserting the PPE portioninto the Z5 byte in accordance with standard TCM, the PPE portion may beinserted into an unused portion of the path overhead portion, which mayinclude either the Z3 or the Z4 byte of a Synchronous Optical NETwork(SONET) path overhead portion, for example. Advantageously, by selectingan unused portion of the path overhead for insertion of the PPE,transparency of the channel is further improved, as the customersability to use other parts of the path overhead, such as the Z5 byte forexample, is not compromised. This is particularly advantageous if thecustomer wishes to perform Tandem Connection Monitoring of thecustomer's own facilities using the Z5 byte (also referred to as the N1byte when used for TCM), as the insertion of the PPE portion into the Z3or Z4 byte would therefore not over-write the customer's own TCMinformation stored in the Z5 byte.

A further aspect of the invention relates to a computer-readable mediumfor providing codes for directing a processor circuit to cause the abovemethod to be carried out. Similarly, another aspect provides a signalembodied in a carrier wave, the signal including code segments fordirecting a processor circuit to cause the method to be carried out. Afurther aspect relates to an apparatus for providing clear channelaccess on a network, the apparatus including provisions for carrying outthe method.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiments of the invention,

FIG. 1 is a block diagram of an apparatus for providing clear channelaccess on a network, according to a first embodiment of the invention;

FIG. 2 is a block diagram of an apparatus for providing clear channelaccess on a network, according to a second embodiment of the invention;

FIG. 3 is a block diagram of a system for providing clear channel accesson a network, according to a third embodiment of the invention;

FIG. 4 is a block diagram of a transport control subsystem (TCS) of afirst network element shown in FIG. 3;

FIG. 5 is a block diagram of a transport control subsystem (TCS) of asecond network element shown in FIG. 3;

FIG. 6 is a flowchart of a previous transport overhead (PTOH) andprevious path error (PPE) insertion thread executed by the TCS shown inFIG. 4;

FIG. 7 is a fragmented tabular representation of a communication signalas modified in response to execution of the PTOH and PPE insertionthread shown in FIG. 6; and

FIG. 8 is a flowchart of a transport overhead modification threadexecuted by the TCS shown in FIG. 5.

DETAILED DESCRIPTION

Referring to FIG. 1, an apparatus for providing clear channel access ona network 20 according to a first embodiment of the invention is showngenerally at 22. The apparatus 22 includes a processor circuit 24configured to insert into a communication signal 26 received at anetwork element 28, a previous transport overhead (PTOH) portion 30indicative of transport overhead contents 32 of the communication signal26 prior to arrival at the network element 28, and a previous path error(PPE) portion 34 indicative of path errors 36 present in thecommunication signal 26 at the network element 28. The apparatus 22further includes a transmitter 38 in communication with the processorcircuit 24, operable to transmit the communication signal 26 to a remotedevice 40.

Referring to FIG. 2, an apparatus for providing clear channel access ona network 42 according to a second embodiment of the invention is showngenerally at 44. The apparatus 44 includes a receiver 46 operable toreceive a communication signal 48 from a remote network element 50. Thecommunication signal 48 includes a previous transport overhead (PTOH)portion 52 indicative of transport overhead contents of thecommunication signal 48 prior to arrival at the remote network element50, and also includes a previous path error (PPE) portion 54 indicativeof path errors present in the communication signal at the remote networkelement 50. The apparatus 44 further includes a processor circuit 56 incommunication with the receiver 46. The processor circuit 56 isconfigured to modify a transport overhead portion shown generally at 58of the communication signal 48 in response to the PTOH and PPE portions52 and 54.

System

Referring to FIGS. 1, 2 and 3, a system for providing clear channelaccess on a network 60 according to a third embodiment of the inventionis shown generally at 62 in FIG. 3. The system 62 includes a firstapparatus, which in this embodiment is a first network element 64similar to the apparatus shown at 22 in FIG. 1, as well as a secondapparatus, which in this embodiment is a second network element 66similar to the apparatus shown at 44 in FIG. 2. The first and secondnetwork elements are in communication with each other over the network60.

Referring to FIG. 3, in this embodiment, the network 60 includes anoptical network in accordance with Synchronous Optical NETwork (SONET)standards, although alternatively, other types of networks may besubstituted. More particularly, in this embodiment the network 60 is anOptical Carrier (OC)-192 network, and includes a plurality of networkelements interposed between the network elements 64 and 66, such asthose shown at 68 for example. In this embodiment the interposed networkelements include pluralities of both section terminating equipment andline terminating equipment devices.

The first network element 64 in the present embodiment includes a firstLine Terminating Equipment (LTE) 70, having various capabilities such ascross-connecting, add/drop multiplexing, etc. The first LTE 70 is incommunication with an OC-48 optical pipe 72, over which it receives acommunication signal shown generally at 74, that is to be multiplexedonto the OC-192 network 60, from a first customer equipment 76.

The first network element 64 includes a processor circuit showngenerally at 78, configured to insert into the communication signal 74received at the network element 64, a previous transport overhead (PTOH)portion 80 indicative of transport overhead contents 82, which in thisembodiment are transport overhead parity error portions, of thecommunication signal 74 prior to arrival at the network element 64, anda previous path error (PPE) portion 84 indicative of path errors 86present in the communication signal 74 at the network element 64. Thefirst network element 64 further includes a transmitter 87 incommunication with the processor circuit 78 and operable to transmit thecommunication signal 74 to a remote device, which in this embodiment isthe second network element 66.

In this embodiment, the processor circuit 78 includes a firsttransmit/receive overhead processor (TROHP) 88, a first SynchronizationDriver Receiver (SYDR) device 90 and a first transport control subsystem(TCS) 92. More particularly, in this embodiment the TROHP 88 includes aTROHP4 Application Specific Integrated Circuit (ASIC), and the SYDRdevice 90 includes four individual SYDR4 TriFEC ASICs, manufactured byNortel Networks Limited of Montreal, Canada. Generally, the TROHP4 ASICis a section/line overhead processor operable to monitor and extractsection and line error counts along with the other section and lineoverhead portions of a Synchronous Transport Signal (STS) communicationsignal. In this regard, the TROHP 88 includes a PM binning register 89for binning or accumulating section (B1) parity errors, and a signaldegrade binning register 91 for binning or accumulating line (B2) parityerrors. Similarly, the SYDR device 90 processes path errors and pathoverhead, and also acts as a path data/clock synchronizer for handlingtiming discrepancies. Alternatively, however, other suitable circuitsmay be substituted.

The transmitter 87 includes a multiplexer module, a laser modulator anda laser (not shown). The first network element 64 also contains elements(not shown) that are well-known components of such network elements andare therefore omitted in FIG. 3 for clarity.

In this embodiment, the second network element 66 includes a second LineTerminating Equipment (LTE) 94, having various capabilities similar tothose of the first LTE 70, including cross-connecting, add/dropmultiplexing, etc. The second LTE 94 is in communication with a secondOC-48 optical pipe 96, over which it transmits a modification of thecommunication signal 74, to a second customer equipment 98.

The second network element 66 includes a receiver 100 operable toreceive a communication signal from a remote network element, which inthis embodiment includes the communication signal 74 received from thefirst network element 64, and thus, the communication signal includesthe PTOH portion 80 and the PPE portion 84. The second network element66 further includes a processor circuit 102 in communication with thereceiver 100. The processor circuit 102 is configured to modify atransport overhead portion shown generally at 104 of the communicationsignal 74 in response to the PTOH and PPE portions 80 and 84.

More particularly, in this embodiment the processor circuit 102 includesa second TROHP 106, a second SYDR device 108 and a second TCS 110,similar to those of the first processor circuit 78, the SYDR device 108comprising four individual SYDR4 ASICs, for example. Also in thisembodiment, the receiver 100 includes a demultiplexer module (notshown). The second network element 66 also contains elements (not shown)that are well-known components of such network elements and aretherefore omitted in FIG. 3 for clarity.

In this embodiment, a Tandem Connection Maintenance (TCM) connection isestablished between the first and second network elements 64 and 66, asdefined in accordance with The American National Standards Institute(ANSI) standard T1.105.05-1994, “Synchronous Optical Network (SONET):Tandem Connection Maintenance”, which is incorporated herein byreference. Thus, in the present embodiment, the first network element 64acts as a tandem connection originating element and the second networkelement 66 acts as a tandem connection terminating element.

In this regard, although one-way communication signals are primarilydescribed herein for ease of illustration, it will be appreciated thatembodiments of the invention are equally applicable to bi-directionalcommunication, in which each one of the first and second networkelements 64 and 66 performs the functions of the first network element64 in the transmit direction, and the functions of the second networkelement 66 in the receive direction.

First Processor Circuit TCS

Referring to FIGS. 3 and 4, the first TCS of the first processor circuit78 is shown generally at 92 in FIG. 4. In this embodiment, the first TCS92 includes a first microcontroller 120, in communication with a firstprogram memory 121, a first working memory 124, and a first input/output(I/O) device 126 via which the TCS 92 is in communication with the TROHP88, the SYDR device 90, and with other components of the network element64.

The program memory 121, which in this embodiment includes a non-volatilememory such as a FLASH or EEPROM for example, stores various routines,subroutines and threads for execution by the microcontroller 120.

Generally, in the present embodiment the program memory 121 acts as acomputer-readable medium for providing codes for directing the processorcircuit 78 to insert into the communication signal 74 received at thenetwork element 64, the previous transport overhead (PTOH) portion 80indicative of the transport overhead contents 82 of the communicationsignal 74 prior to arrival at the network element 64, and the previouspath error (PPE) portion 84 indicative of the path errors 86 present inthe communication signal 74 at the network element 64, and to transmitthe communication signal 74 to a remote device, which in this embodimentis the second network element 66. More particularly, in this embodimentthe program memory stores a PTOH and PPE insertion thread 128, fordirecting the processor circuit 78 to insert the PTOH portion 80 and PPEportion 84 shown in FIG. 3 into the communication signal 74.Alternatively, however, it will be appreciated that the PTOH and PPEinsertion thread 128 may be omitted if hardware components of theprocessor circuit 78, such as the TROHP 88 and/or the SYDR device 90 forexample, are pre-configured to insert the PTOH and/or PPE portions intothe communication signal 74. Similarly, any other suitablecomputer-readable medium may be substituted.

In addition, it will be appreciated that the program memory 121 merelyprovides one way of generating a signal embodied in a carrier wave, thesignal including code segments for directing the processor circuit toinsert the PTOH and PPE portions 80 and 84 into the communication signalin the above manner and to transmit the communication signal to a remotedevice. Alternatively, other types of media, signals, or ways ofgenerating such signals may be substituted.

In this embodiment, the program memory 121 also includes a PTOH headervalues register 122, in which unique PTOH header values are stored, asdiscussed in greater detail below. The program memory 121 also stores anindex look-up table 123, for use by the microcontroller in executing thePTOH and PPE insertion thread. In addition, the program memory 121stores routines (not shown, not part of this invention) for directingthe processor circuit 78 to execute conventional network elementfunctionality such as LTE, ADM and cross-connect functionality, forexample.

In this embodiment, the PTOH and PPE insertion routine 128 configuresthe microcontroller 120 to define various registers in the first workingmemory 124, including a section errors register 125 for storing anindication of accumulated section (B1) parity errors in the incomingcommunication signal 74 received at the first network element, a lineerrors register 127 for storing an indication of accumulated line (B2)parity errors in the incoming communication signal, and a PTOH register129 for storing the PTOH portion 80 that is to be inserted into thetransport overhead portion 104 of the communication signal 74.

Second Processor Circuit TCS

Referring to FIGS. 3 and 5, the second TCS of the second processorcircuit 102 is shown generally at 110 in FIG. 5. In this embodiment, thesecond TCS 110 includes a second microcontroller 130, in communicationwith a second program memory 132, a second working memory 134, and asecond input/output (I/O) device 136 via which the TCS 110 is incommunication with the TROHP 106, the SYDR device 108, and with othercomponents of the network element 64.

The program memory 132, which in this embodiment includes a non-volatilememory such as a FLASH or EEPROM for example, stores various routines,subroutines and threads for execution by the microcontroller 130.

Generally, the program memory 132 acts as a computer-readable medium forproviding codes for directing the processor circuit 102 to modify thetransport overhead portion 104 of the communication signal 74 receivedfrom the network element 64, in response to the PTOH portion 80 of thecommunication signal indicative of the transport overhead contents 82 ofthe communication signal prior to arrival at the remote network element64, and the PPE portion 84 of the communication signal indicative of thepath errors 86 present in the communication signal at the remote networkelement. More particularly, in this embodiment the program memory 132stores a transport overhead modification thread 138, for directing theprocessor circuit 102 to modify the transport overhead portion 104 ofthe communication signal 74 shown in FIG. 3, in response to the PTOHportion 80 and the PPE portion 84. Alternatively, however, any othersuitable computer-readable medium may be substituted.

In addition, it will be appreciated that the program memory 132 merelyprovides one way of generating a signal embodied in a carrier wave, thesignal including code segments for directing the processor circuit tomodify the transport overhead portion in response to the PTOH and PPEportions in the above manner. Alternatively, other types of media,signals, or ways of generating such signals may be substituted.

In this embodiment, the program memory 132 also stores index look-uptables shown generally at 139, which in this embodiment include aforward index look-up table 140, a reverse index look-up table 142, anda hardware configuration look-up table 144, for use by themicrocontroller 130 in executing the transport overhead modificationthread 138.

The program memory 132 also stores routines (not shown, not part of thisinvention) for directing the processor circuit 102 to executeconventional network element functionality such as LTE, ADM andcross-connect functionality, for example.

The routines stored in the program memory 132 direct the processorcircuit 102 to define various registers in the second working memory134, including a differences register 202, a sum of differences register203, a PTOH register 204 including a previous section overhead field 205and a previous line overhead field 206, an outgoing section errorregister 207 and an outgoing line error register 208. Such registers arediscussed in greater detail below.

Operation

Referring to FIG. 3, in this embodiment the first and second networkelements 64 and 66 are configured, via a manual user provisioningoperation, to transport the communication signal 74 received from thefirst customer equipment 76 to the second customer equipment 98 as aclear channel. (Alternatively, however, if desired, the first and secondnetwork elements may be quickly and easily configured by a similar userprovisioning operation to transport the communication signal 74 in aconventional (non-clear) manner.)

First Network Element

Referring to FIGS. 3, 4, 6 and 7, the PTOH and PPE insertion thread isshown generally at 128 in FIG. 4. Generally, the PTOH and PPE insertionthread 128 configures the processor circuit 78 to insert the PTOH andPPE portions 80 and 84 into the communication signal 74 shown in FIG. 3,and to transmit the communication signal to a remote device, which inthis embodiment is the second network element 66.

The PTOH and PPE insertion thread 128 begins with a first block 150 ofcodes, which generally configures the processor circuit 78 to calculatethe PPE portion 84 in response to path parity errors present in thecommunication signal 74 at the network element 64, and to insert the PPEportion into a path overhead portion of the communication signal, ormore particularly, into an unused portion of the path overhead portion.In this embodiment block 150 first directs the microcontroller 120 ofthe processor circuit 78 to signal the SYDR device 90, in order toconfigure the SYDR device to insert the PPE portion 84 into the unusedpath overhead portion of the communication signal 74.

In this regard, referring to FIGS. 6 and 7, a fragmented representationof the communication signal is shown generally at 74 in FIG. 7. In thisembodiment, the communication signal 74 is an OC-48 SONET communicationsignal, but is illustrated in FIG. 7 with reference to its electricallyequivalent Synchronous Transport Signal (STS)-48 signal. Moreparticularly, the communication signal 74 includes the transportoverhead (TOH) portion shown generally at 104 and a synchronous payloadenvelope (SPE) portion shown generally at 164. The SPE 164 includes apayload portion 166 and a path overhead portion 168.

The transport overhead portion 104 includes a plurality of unusedtransport overhead portions, such as unused transport overhead portionsshown generally at 162 represented by shaded regions in FIG. 7, forexample. Such shaded regions generally correspond to unused time-slotsof transport overhead bytes that are defined only for the first STS-1component of an STS-N signal, and are not defined for the remaining STScomponents of the signal. Thus, it is not necessary to terminate anysuch bytes at section- or line-terminating equipment and accordingly, inthe present embodiment, any section- or line-terminating equipmentdevices interposed on the network 60 between the first and secondnetwork elements 64 and 66, are configured to transparently pass suchunused time-slot bytes. Similarly, the path overhead portion 168includes a plurality of unused path overhead portions, such as aplurality of Z3 bytes shown generally at 174 or a plurality of Z4 bytesshown at 176, for example. Although these unused path overhead portionsare not unused time-slots, they are nevertheless unused, as the Z3 andZ4 bytes are presently unallocated growth bytes of SONET. It will beappreciated that the entire SPE 164 is transparently passed between thenetwork elements 64 and 66, as there is no path terminating equipmentlocated therebetween.

Referring to FIGS. 3 and 7, it will be appreciated that TandemConnection Maintenance (TCM) according to the ANSI T1.105.05-1994standard involves, for each interleaved STS-1 component of an STS-Nsignal frame, calculating the number of path errors 86 shown in FIG. 3.This is achieved by counting the number of parity errors in the SPEportion 164 of the preceding STS-1 component before scrambling, using abit interleaved parity 8 code with even parity. In other words, thenumber of parity errors is counted using the same method that would beused by path originating equipment (not shown) to calculate an initialB3 path error monitoring byte 170 of the path overhead 168. This countednumber of parity errors will differ from the B3 byte 170 if any biterrors in the relevant (preceding STS-1) SPE portion 164 have occurredsince the signal 74 was initially generated and transmitted by thecustomer's path originating equipment. Accordingly, the contents of theB3 byte 170 of a given STS-1 component are subtracted from the number ofparity errors counted over the previous STS-1 component, to yield thenumber of path errors 86 present in the previous STS-1 component of thecommunication signal 74 at the network element 64, or in other words,the number of new parity errors that have arisen in that componentbetween the time the B3 byte was generated at the customer's pathoriginating equipment, and the time the signal 74 arrived at the firstnetwork element 64. In this embodiment, it is this latter resultingnumber of path errors 86 that is to be inserted into the communicationsignal 74 as the previous path error (PPE) portion 84 shown in FIG. 3.

TCM further involves storing the number of path errors 86 in bits 1-4 ofa Z5 byte 172 in the path overhead portion 168. Finally, TCM involvescompensating the B3 byte 170, by adjusting the value of the B3 byte totake into account the parity change resulting from writing the number ofpath errors 86 to the Z5 byte 172.

If desired, block 150 may simply direct the microcontroller 120 of theprocessor circuit 78 to configure the SYDR device 90 to perform theabove-noted conventional TCM steps to insert the number of path errors86 into the first 4 bits of the Z5 byte 172, with the result that the Z5byte 172 becomes the PPE portion 84, and to compensate the B3 byte 170for the parity change resulting from the insertion.

Advantageously, however, in the present embodiment block 150 differsfrom conventional TCM, in that it configures the processor circuit 78 toinsert the PPE portion 84 into at least one of a Z3 and a Z4 byte of aSynchronous Optical NETwork (SONET) path overhead portion. Moreparticularly, in this embodiment block 150 directs the processor circuit78 to configure the SYDR device 90 to write the number of path errors 86to a Z3 byte 174 of the path overhead portion 168 rather than to the Z5byte 172. Thus, in this embodiment the PPE portion 84 includes the Z3byte 174 following such writing by the SYDR device. In this regard, itis possible that the customer, from whom the OC-48 communication signal74 is originating, may wish to use the Z5 byte 172 to perform TCM acrossthe customer's own equipment, such as between the first and secondcustomer equipment 76 and 98, for example. Therefore, by writing thenumber of path errors 86 to the Z3 byte 174 which is presently an unusedSONET growth byte, the SPE 164 is effectively transparently passed tothe customer (apart from the unused Z3 byte), without interfering withthe customer's ability to use path overhead bytes such as Z5 for TCM,for example. Alternatively, block 150 may direct the processor circuit78 to configure the SYDR device 90 to write the number of path errors 86to a different byte, such as a Z4 byte 176 which is also presently anunused growth byte, resulting in similar advantages.

In any of the above three variations (i.e., writing the number of patherrors 86 to Z5, Z3 or Z4), block 150 further configures the processorcircuit 78 to adjust a path parity error portion of the communicationsignal to compensate for insertion of the PPE portion into the pathoverhead portion. More particularly, block 150 directs the processorcircuit 78 to configure the SYDR device 90 to compensate the B3 byte 170for the parity change resulting from writing the number of path errors86 to the path overhead portion 168, in accordance with the TCM standardB3 compensation equation.

Referring to FIGS. 3 and 6, once initially configured in the abovemanner at block 150, the SYDR device 90 continues to continuously insertthe PPE portion 84 into successive frames of the communication signal 74received at the network element 64. Thus, in this embodiment the PPEinsertion is performed by hardware, namely, by the SYDR device.Alternatively, however, software methods, such as a software simulationof TCM to effectively insert the PPE portion for example, may besubstituted if desired.

Referring to FIGS. 3, 4, 6 and 7, block 180 then configures theprocessor circuit 78 to insert the PTOH portion 80 into the transportoverhead portion 104 of the communication signal, or more particularly,into the unused portion 162 of the transport overhead portion 104. Inthis embodiment the unused portion is an unused time-slot of thetransport overhead portion 104, such as those described earlier hereinin connection with the shaded regions of FIG. 7, for example. Moreparticularly still, in this embodiment the unused time-slot is the STS#9time-slot of the K2 byte shown in FIG. 7, as discussed in greater detailbelow. Alternatively, however, one or more other portions of thetransport overhead portion may be substituted, although unused portionsare preferred for this purpose.

In this embodiment, block 180 achieves such insertion by first directingthe microcontroller 120 of the processor circuit 78 to cooperate withthe TROHP 88 to insert the PTOH portion 80 into the transport overheadportion 104 of the communication signal 74. In this embodiment the PTOHportion 80 is generated and inserted into the transport overhead portion104 by software methods, or more particularly, in response to acontinuous execution of block 180 by the microcontroller 120. Moreparticularly, in this embodiment block 180 configures the processorcircuit 78 to calculate the PTOH portion 80 in response to the previoustransport overhead contents 82 of the communication signal 74 prior toits arrival at the first network element 64, over a previous 100millisecond interval, rather than passing such transport overheadcontents directly. Alternatively, however, if suitable hardwarecomponents are available, block 180 may direct the microcontroller toconfigure such hardware components to continuously insert the PTOHportion 80.

In this embodiment, to generate the PTOH portion 80, block 180 firstconfigures the processor circuit 78 to count a number of transportparity errors present in the communication signal 74 prior to itsarrival at the network element. More particularly, block 180 directs themicrocontroller 120 to sample the contents of the PM binning register 89of the TROHP 88, once per second, to store a value equal to one-tenth ofthe contents of the PM binning register 89 in the section errorsregister 125 of the first working memory 124, and to reset the contentsof the PM binning register. Thus, as the PM binning register of theTROHP 88 stores an accumulated number of section parity errors in theincoming communication signal 74 received at the first network element64, the contents of the section errors register 125 at any given timerepresents the average number of section parity errors in the incomingcommunication signal 74 in each of the ten 100 ms sub-intervals in theone-second interval preceding the most recent sampling of the PM binningregister 89. As the PM binning register contents are incremented by theTROHP 88 in response to parity discrepancies between the entire contentsof a given STS-48 frame of the communication signal 74 and the B1 byteof the subsequent STS-48 frame, the contents of the section errorsregister 125 are, in that sense, indicative of the previous transportoverhead contents 82, or more particularly of the section (B1) parityerror bytes of the communication signal 74.

Each time a value equal to one-tenth the sampled contents of the PMbinning register 89 is stored in the section errors register 125 in theabove manner, block 180 further directs the microcontroller 120 to storea value corresponding to the contents of the section errors register ina section field of the PTOH register 129, for insertion into thetransport overhead portion of the communication signal 74. Moreparticularly, if the contents of the section errors register 125 areless than 126, block 180 configures the processor circuit 78 toeffectively set the PTOH portion 80 equal to the counted number oftransport parity errors (in this example, section parity errors) presentin the communication signal prior to its arrival at the first networkelement 64. More particularly still, block 180 directs themicrocontroller 120 to store such contents of the section errorsregister 125, as a raw section error value, in the section field of thePTOH register 129. If desired, this value may be further “massaged”, byrounding it up to the nearest multiple of eight for example, prior tostoring it in the section field of the PTOH register. Such massaging maybe useful if the TROHP at the second network element 66 is constrainedin its ability to corrupt certain numbers of bits in the outgoingcommunication signal.

However, if the contents of the section errors register 125 are greaterthan or equal to 126 (i.e. if the most recently sampled PM binningregister 89 contents were greater than or equal to 1255), block 180configures the processor circuit 78 to effectively set the PTOH portion80 equal to an index value indicative of the counted number of transportparity errors (in this example, section parity errors) present in thecommunication signal 74 prior to its arrival at the first networkelement 64. More particularly, block 180 directs the microcontroller 120to use the contents of the section errors register 125 to search theindex look-up table 123, to locate an index number corresponding to suchcontents, and to store the located index number in the section field ofthe PTOH register 129. In this regard, Table 1 illustrates an exemplaryindex look-up table. The middle column showing equivalent bit errorrates (BERs) is not used in the present embodiment, but is shown belowfor illustrative purposes.

TABLE 1 (Exemplary Index Look-up Table 123) X = contents of sectionerrors register 125; -or- Equivalent bit error rate of Index X =contents of line errors incoming OC-48 comm. value register 127 signal74 (rounded) (hex)   126 ≦ X ≦ 186 5 × 10⁻⁷ 0D   187 ≦ X ≦ 622 1 × 10⁻⁶0E   623 ≦ X ≦ 1866 5 × 10⁻⁶ 0F  1867 ≦ X ≦ 6220 1 × 10⁻⁵ 10  6221 ≦ X ≦18662 5 × 10⁻⁵ 11  18663 ≦ X ≦ 62207 1 × 10⁻⁴ 12  62208 ≦ X ≦ 186623 5 ×10⁻⁴ 13 186624 ≦ X ≦ 622079 1 × 10⁻³ 14 622080 ≦ X AIS FF

Upon locating the corresponding index number, the microcontroller 120 isdirected by block 180 to store the index number in the section field ofthe PTOH register 129. Alternatively, however, other index tables, ormore broadly, any other suitable approximation method may be used toapproximate such large error values with a single byte, or as a furtheralternative, more than one byte of stolen overhead may be used totransport such large error values, if desired.

Similarly, in this embodiment block 180 also configures the processorcircuit 78 to count a second number of transport parity errors, or moreparticularly, line parity errors, present in the communication signal 74prior to its arrival at the first network element 64. To achieve this,block 180 directs the microcontroller 120 to effectively sample thecontents of the signal degrade binning register 91 of the TROHP 88 onceevery 100 milliseconds, to store the contents of the signal degradebinning register 91 in the line errors register 127 of the first workingmemory 124, and to reset the contents of the signal degrade binningregister. (If desired, the signal degrade binning register 91 may besampled more frequently, such as three times over each 100 ms intervalfor example, and the sampled contents may be accumulated in a separateregister (not shown) which is copied to the line errors register 127every 100 ms and is then reset to continue accumulating such morefrequent samples.) Thus, as the signal degrade binning register 91 ofthe TROHP 88 stores an accumulated number of line parity errors in theincoming communication signal 74 received at the first network element64, the contents of the line errors register 127 at any given timerepresents the total number of line parity errors in the incomingcommunication signal 74 in the 100 ms interval preceding the most recentsampling of the signal degrade binning register. As the signal degradebinning register contents are incremented by the TROHP 88 in response toparity discrepancies between the line overhead and synchronous payloadenvelope of a given STS-1 component of an STS-48 frame of thecommunication signal 74 and the B2 byte of the subsequent STS-1component, the contents of the line errors register 127 are, in thatsense, indicative of the previous transport overhead contents 82, ormore particularly of the line (B2) parity error bytes of thecommunication signal 74.

Each time the line errors register 127 is updated in the above manner inresponse to the sampled contents of the signal degrade binning register91 over a given 100 ms interval, block 180 further directs themicrocontroller 120 to store a value corresponding to the contents ofthe line errors register in a line field of the PTOH register 129, forinsertion into the transport overhead portion of the communicationsignal 74. More particularly, if the contents of the line errorsregister 127 are less than 126, block 180 configures the processorcircuit 87 to effectively set the PTOH portion 80 equal to the countednumber of transport parity errors (in this example, line parity errors)present in the communication signal 74 prior to arrival at the firstnetwork element 64. To achieve this, block 180 directs themicrocontroller 120 to store such contents of the line errors register127, as a raw line error value, in the line field of the PTOH register129. As with the section field, the contents of the line errors register127 may be “massaged” if desired, by rounding such contents up to thenearest multiple of eight for example, prior to storing such contents inthe line field of the PTOH register 129.

However, if the contents of the line errors register 127 are greaterthan or equal to 126, block 180 configures the processor circuit 78 toeffectively set the PTOH portion 80 equal to an index value indicativeof the counted number of transport parity errors (in this example, lineparity errors). More particularly, block 180 directs the microcontroller120 to use the contents to search the index look-up table 123 shown inTable 1 above, to locate an index number corresponding to the value, andto store the located index number in the section field of the PTOHregister 129.

Upon locating the corresponding index number, the microcontroller 120 isdirected by block 180 to store the index number in the line field of thePTOH register 129.

In addition, block 180 directs the microcontroller 120 to cooperate withthe TROHP to insert the contents of the PTOH register 129 into theunused portion 162 of the transport overhead portion of thecommunication signal 74. More particularly, in this embodiment, block180 directs the microcontroller 120 to effectively transmit the contentsof the section field and the line field of the PTOH register 129 tentimes per second, in a sufficiently slow manner as to be detectable bysoftware at the second network element 66, as follows.

In this regard, block 180 configures the processor circuit 78 to inserta PTOH header value in the communication signal, preceding the PTOHportion 80, to identify the PTOH portion. More particularly, within eachsuccessive 100 ms interval, block 180 directs the microcontroller 120 tocooperate with the TROHP 88 to insert a unique B2 header byte into theSTS#9 time-slot of the K2 byte for a first sub-interval of 25 ms (or inother words, for 200 successive frames). In this embodiment, the uniqueB2 header byte is obtained from the PTOH header values register 122, andserves to indicate to the second network element 66 that the byte thatis to follow in the next 25 ms sub-interval is a previous transportoverhead portion, indicative of previous line (B2) parity error contentsof the communication signal 74 prior to its arrival at the first networkelement 64. Block 180 then directs the microcontroller 120 to cooperatewith the TROHP 88 to insert the contents of the line field of the PTOHregister 129 into the STS#9 time-slot of the K2 byte for a secondsub-interval of 25 ms (i.e. for the next 200 successive data frames).This allows sufficient time for the contents of the line field to bedetected at the second network element 66.

Similarly, block 180 then directs the microcontroller 120 to cooperatewith the TROHP 88 to insert a unique B1 header byte into the STS#9time-slot of the K2 byte for a third sub-interval of 25 ms (i.e. for thenext 200 successive frames). In this embodiment, the unique B1 headerbyte is obtained from the PTOH header values register 122, and serves toindicate to the second network element 66 that the byte that is tofollow in the next 25 ms sub-interval is a previous transport overheadportion, indicative of previous section (B1) parity error contents ofthe communication signal 74 prior to its arrival at the first networkelement 64. Block 180 then directs the microcontroller 120 to cooperatewith the TROHP 88 to insert the contents of the section field of thePTOH register 129 into the STS#9 time-slot of the K2 byte for a fourthsub-interval of 25 ms (i.e. for the next 200 successive data frames).

In this embodiment, block 180 continues to direct the microcontroller120 to insert the PTOH portion 80, or more particularly, the contents ofthe PTOH register 129, into the STS#9 time-slot of the K2 byte of thecommunication signal 74 in the above manner, repeating the abovefour-part cycle once every 100 ms. Similarly, block 180 continues todirect the microcontroller to sample the contents of the PM binningregister 89 once per second and to sample the contents of the signaldegrade binning register 91 effectively once every 100 ms (oralternatively, more often), and to update the contents of the sectionerrors register 125, the line errors register 127, and the section andline fields of the PTOH register 129, thereby updating the PTOH portion80 inserted into the communication signal 74, as described above. Block180 is thus executed indefinitely, as a thread, by the microcontroller120.

The communication signal 74, which in this embodiment is an OC-48signal, including the PTOH and PPE portions 80 and 84, isbyte-interleave-multiplexed by the network element 64 into an OC-192signal which is transmitted on the network 60 by the transmitter 87 ofthe first network element 64. The PTOH portion 80 and the PPE portion 84are transparently passed through any network elements 68 such as LTEsfor example on the network 60, as the PPE portion is stored in thesynchronous payload envelope 164 which is always carried transparentlythrough non-path-terminating elements, and the intervening networkelements 68 are configured to transparently pass the unused portion 162(or more particularly, the unused OC#9 time-slot of the K2 bytes) of thetransport overhead portion 104, in which the PTOH portion 80 is stored.In this embodiment, the communication signal 74 is multiplexed into anOC-192 signal comprising other communication signals that are not to beprovided as clear channels. Alternatively, however, as no specialdedicated facilities are required in the present embodiment, any desiredmixture of clear channel and non-clear channel communication signals maybe multiplexed together as desired.

Second Network Element

Referring to FIGS. 3, 5, 7 and 8, upon arrival at the second networkelement 66 of the communication signal 74, as a multiplexed component ofthe OC-192 signal transmitted by the first network element 64, thecommunication signal 74 is demultiplexed from the OC-192 signal.Advantageously, in the present embodiment, as dedicated clear channelfacilities are not required, the communication signal 74 isdemultiplexed from the OC-192 signal in the same manner that non-clearchannels are customarily demultiplexed, without the need to demultiplexthe entire OC-192 signal and re-multiplex the signal components that arenot being dropped off at the second network element 66. At the secondnetwork element 66, as the communication signal 74 is demultiplexed, theprocessor circuit 102 of the second network element commences executionof the transport overhead modification thread 138 shown in FIG. 8.

The transport overhead modification thread begins with a first block 200of codes, which configures the processor circuit 102 to calculate adifference between path errors present in the communication signal 74,and the path errors 86 that were present in the communication signal atthe remote first network element 64. More particularly, in thisembodiment, block 200 configures the processor circuit 102 to calculatesuch a difference for each payload portion having valid path overhead inthe communication signal 74. Thus, for example, if the communicationsignal 74 includes a plurality of STS-1 component signals, block 200directs the microcontroller 130 to configure the processor circuit 102,or more particularly the SYDR device 108, to calculate, for each of theSTS-1 component signals, a difference between path errors present in theSTS-1 component signal and the path errors 86 that were present in theSTS-1 component signal at the remote first network element 64.Alternatively, if the communication signal 74 includes a plurality ofconcatenated payloads in respective concatenated STS-nc componentsignals, it will be appreciated that only the first STS-1c subcomponentof each STS-nc component signal (for example, STS#1, #13, #25 and #37 ifthe STS-48 communication signal 74 includes four STS-12c componentsignals) will have valid path overhead and accordingly, block 200directs the microcontroller to configure the SYDR device 108 tocalculate such a difference value only for the first such STS-1csubcomponent of each STS-nc component signal. For ease of illustration,however, the following discussion focuses primarily on thenon-concatenated example in which the communication signal 74 includesnon-concatenated STS-1 component signals.

In this regard, it will be appreciated that conventional TCMfunctionality at a tandem connection termination equipment (TCTE) suchas the second network element 66, involves, for each STS-1 component ofthe communication signal 74, re-calculating the number of parity errorspresent in the SPE portion 164 of the previous STS-1 component. Thecontents of the B3 byte 170 of the current STS-1 component are thensubtracted from this re-calculated number of parity errors over theprevious component, to yield the number of path errors present in theprevious STS-1 component of the signal 74 upon its arrival at the TCTE,or in other words, the number of bit errors that have occurred in theSPE 164 of the previous STS-1 component since the communication signal74 was initially generated and transmitted by the customer's pathoriginating equipment. A difference between this number of path errorspresent in the signal 74 at the TCTE, and the number of path errors 86that were present in the signal 74 at the tandem connection originating(TCO) point (in this embodiment, the first network element 64) is thencalculated. In this regard, it will be recalled that the number of patherrors 86 present at the TCO is stored in the first 4 bits of the Z5byte 172 according to conventional TCM, and therefore, this differenceis calculated by subtracting the contents of the first 4 bits of the Z5byte from the number of path errors present in the signal 74 at theTCTE. The magnitude of this difference represents the number of new patherrors that occurred in the previous STS-1 SPE 164 along its journeybetween the TCO and the TCTE. The TCTE then records this value fornetwork monitoring purposes, resets the first 4 bits of the Z5 byte 172to zeroes, and compensates the B3 byte 170 to account for the paritychange resulting from this resetting.

Accordingly, referring to FIGS. 3, 5, 6, 7 and 8, if block 150 of thePTOH and PPE insertion thread 128 shown in FIG. 6 configured theprocessor circuit 78 to insert the PPE portion 84 into the Z5 byte 172in the same manner as a conventional TCO element, then similarly, block200 of the transport overhead modification thread 138 would configurethe processor circuit 102 to calculate such difference values in thesame manner as a conventional TCTE.

However, as noted, in the present embodiment block 150 of the PTOH andPPE insertion thread 128 configures the processor circuit 78 to insertthe number of path errors 86 into the Z3 byte 174 (or alternatively, theZ4 byte 176) rather than the Z5 byte 172. Accordingly, in the presentembodiment block 200 of the transport overhead modification thread 138directs the microcontroller 130 to configure the SYDR device 108 tocalculate the difference values by, for each STS-1 component,subtracting the contents of the first 4 bits of the Z3 byte 174 (oralternatively, of the Z4 byte 176) rather than of the Z5 byte 172, fromthe number of path errors present in the previous STS-1 component at thesecond network element 66. Similarly, rather than resetting the first 4bits of the Z5 byte, block 150 configures the processor circuit 102 toreset the first 4 bits of the Z3 byte 174 (or alternatively the Z4 byte176), and to compensate the B3 byte for the resulting parity change.Advantageously, therefore, if the customer who supplied thecommunication signal 74 is attempting to use the Z5 (or N1) bytes 172 toperform TCM between points on his own facilities, the customer's abilityto do so will not be destroyed, as the present embodiment does notinvolve any modifications to the Z5 byte at all over the network 60.

Once configured in this manner at block 200, the SYDR device 108continuously calculates difference values in the above manner andcontinuously provides signals representing such values to themicrocontroller 130. Thus, in this embodiment the TCM calculations atthe second network element 66 are also performed by hardware, namely theSYDR device 108. Alternatively, however, software-simulated TCM may besubstituted if desired.

Referring to FIGS. 3, 5, 7 and 8, block 210 then configures theprocessor circuit 102 to calculate a sum of the differences of each ofthe component signals. To achieve this, block 210 directs themicrocontroller 130 to receive the signals from the SYDR device 108representing the difference value (described above in connection withblock 200) for each of the 48 STS-1 components of each STS-48 signal,and to store such values in the differences register 202 in the secondworking memory 134. Block 210 further directs the microcontroller 130 tomaintain a sum of the difference values in the sum of differencesregister 203. In this embodiment, the difference values are collectedfrom the SYDR device 108 once per second, and a sum of differences iscalculated over each such collected set of difference values. The sum ofdifferences, which represents the total number of new path parity errorsproduced in the communication signal 74 during the course of its voyagebetween the first and second network elements 64 and 66 over aone-second interval, is then divided by ten, to produce an average sumof differences value over each 100 ms sub-interval. (It will be recalledthat this 100 ms sub-interval is the same duration of sub-interval asthat over which the PTOH portion 80, which was inserted into thecommunication signal 74 at the first network element 64, was calculated.Accordingly, it is desirable to divide the sum of differences by ten inthis manner so that the sum of differences is effectively calculatedover the same duration of time interval as the PTOH portion 80.) Theresult of such division is stored in the sum of differences register203. Thus, in the present embodiment, the contents of the sum ofdifferences register 203 are updated once every second, but correspondto an average sum of difference values over each 100 ms time interval inthe preceding second. Alternatively, however, the sum of differencesregister 203 may be updated more or less frequently if desired.

In addition, referring to FIGS. 3 to 8, block 210 directs themicrocontroller 130 to cooperate with the TROHP 106 to begincontinuously extracting the PTOH portion 80 stored in the unusedtransport overhead portion 162 of the communication signal 74. In thisembodiment, such continuous monitoring and extraction of the PTOHportion 80 is achieved in a 100 ms four-part cycle consisting of four 25ms sub-intervals, corresponding to the four-part cycle described abovein connection with block 180 at the first network element 64.

In this regard, block 210 first directs the microcontroller 130 tocooperate with the TROHP 106 to monitor the contents of the STS#9time-slot of the K2 byte of the communication signal 74. Upon detectingthe presence of the unique B2 header value in this STS#9 time-slot,block 210 directs the microcontroller 130 to continue to monitor the K2STS#9 time-slot of the communication signal until a byte other than theunique B2 header value is detected. Upon detecting such a byte, if thebyte represents a raw line error value less than 126, as described abovein connection with block 180, block 210 directs the microcontroller 130to copy the byte into the previous line overhead field 206 of the PTOHregister 204. Alternatively, if the detected byte represents a lineerror index value as described in connection with Table 1 above, block210 directs the microcontroller 130 to use the index value byte tosearch the reverse index look-up table 142, to extract an approximatedline error count value corresponding to the detected byte. In thisregard, the reverse index look-up table 142 is similar to a reverse ofthe index look-up table 123 shown in Table 1 above, but provides asingle average or approximated error count value (rather than a range oferror count values) corresponding to each index value. Block 210 thendirects the microcontroller 130 to store the located approximated errorcount value in the previous line overhead field 206 of the PTOH register204.

Block 210 then directs the microcontroller 130 to cooperate with theTROHP 106 to continue monitoring the contents of the STS#9 time-slot ofthe K2 byte of the communication signal 74. Upon detecting the presenceof the unique B1 header value in this STS#9 time-slot, block 210 directsthe microcontroller 130 to continue to monitor the K2 STS#9 time-slot ofthe communication signal until a byte other than the unique B1 headervalue is detected. Upon detecting such a byte, if the byte represents araw section error value less than 126, as described above in connectionwith block 180, block 210 directs the microcontroller 130 to copy thebyte into the previous section overhead field 205 of the PTOH register204. Alternatively, if the detected byte represents a section errorindex value as described in connection with Table 1 above, block 210directs the microcontroller 130 to use the index value byte to searchthe reverse index look-up table 142, to locate and extract anapproximated error count value corresponding to the detected sectionerror index value byte. Block 210 then directs the microcontroller 130to store the located approximated error count value in the previoussection overhead field 205 of the PTOH register 204.

Block 210 further directs the microcontroller 130 to continuouslymonitor the K2 STS#9 time-slot as indicated in the preceding twoparagraphs, to continuously extract the PTOH portion 80, and to storecorresponding line and section error values in the previous line andsection overhead fields 206 and 205 respectively as described above.

Block 210 further configures the processor circuit 102 to modify thetransport overhead portion 104 of the communication signal 74, inresponse to the extracted PTOH and PPE portions 80 and 84. Moreparticularly, in this embodiment block 210 configures the processorcircuit to adjust the transport overhead portion 104 in response to asum of at least some contents of the PTOH portion plus the sum ofdifferences value stored in the sum of differences register 203.

In this embodiment, block 210 achieves this by directing themicrocontroller 130 to add the contents of the sum of differencesregister 203 to the contents of the previous section overhead field 205,and to store the resulting value in the outgoing section error register207. If desired, this resulting value may be “massaged”, by rounding itup to the nearest multiple of eight for example, prior to storing it inthe outgoing section error register 207. As noted, such massaging may beuseful if the TROHP 106 is constrained in its ability to corrupt certainnumbers of bits in the outgoing communication signal.

If the contents of the outgoing section error register 207 are less than126, block 210 directs the microcontroller 130 to use the contents ofthe outgoing section error register 207 to locate a corresponding recordin the hardware configuration look-up table 144, to look up registerconfigurations of the TROHP 106 that must be implemented in order toinvert bits of the B1 section parity error bytes of the transportoverhead portion 104 of the outgoing communication signal 74, toeffectively simulate a bit error count over a 100 ms interval equivalentto the contents of the outgoing section error register 207. In thisregard, it will be appreciated that typical TROHP ASICs will have theability, for diagnostic purposes, to invert certain bits of transportoverhead, such as inverting one bit or all bits, of a single B1 or B2byte, of all B2 bytes in a frame, or of all B1 or B2 bytes over a givennumber of frames, for example. Thus, the particular registerconfiguration addresses and values stored in the hardware configurationlook-up table 144 are dependent upon the make and model of theparticular TROHP ASIC used, and will vary from ASIC to ASIC. Uponlocating the hardware configuration look-up table record correspondingto the contents of the outgoing section error register 207, block 210directs the microcontroller 130 to set the relevant registerconfigurations of the TROHP 106 in accordance with the located record,to invert an appropriate number of bits of an appropriate number of B1section parity error bytes to simulate an outgoing error rate equivalentto the error count value stored in the outgoing section error registerover a 100 ms interval.

Similarly, if the contents of the outgoing section error register 207are greater than or equal to 126, block 210 directs the microcontroller130 to use the contents of the outgoing section error register 207 tolocate a corresponding index value in the forward index look-up table140. In this embodiment, the forward index look-up table 140 isidentical to the index lookup table 123, shown as Table 1 above. Block210 then directs the microcontroller to use the located index value tolook up a corresponding hardware configuration record in the hardwareconfiguration look-up table 144, to look up register configurations ofthe TROHP 106 that must be implemented in order to invert bits of the B1section error bytes of the transport overhead portion 104 of theoutgoing communication signal 74, to effectively simulate a bit errorrate over a 100 ms interval equivalent to the bit error ratecorresponding to the index value that corresponds to the contents of theoutgoing section error register 207 (see Table 1 above). Themicrocontroller is then directed to set such register contents of theTROHP in accordance with the located record.

In addition, block 210 configures the processor circuit 102 to furthermodify the transport overhead portion 104 of the outgoing communicationsignal 74, by adjusting B2 line parity error bytes thereof. Moreparticularly, block 210 directs the microcontroller 130 to add thecontents of the sum of differences register 203 to the contents of theprevious line overhead field 206, and to store the resulting value inthe outgoing line error register 208. As discussed in connection withthe outgoing section error register, if desired, this resulting valuemay be “massaged”, by rounding it up to the nearest multiple of eightfor example, prior to storing it in the outgoing line error register207.

If the contents of the outgoing line error register 208 are less than126, block 210 directs the microcontroller 130 to use the contents ofthe outgoing line error register 208 to locate a corresponding record inthe hardware configuration lookup table 144, to look up registerconfigurations of the TROHP 106 that must be implemented in order toinvert bits of the B2 line parity error bytes of the transport overheadportion 104 of the outgoing communication signal 74, to effectivelysimulate a bit error count over a 100 ms interval equivalent to thecontents of the outgoing line error register 208. Upon locating thehardware configuration look-up table record corresponding to thecontents of the outgoing line error register 208, block 210 directs themicrocontroller 130 to set the relevant register configurations of theTROHP 106 in accordance with the located record, to invert anappropriate number of bits of an appropriate number of B2 line parityerror bytes to simulate an outgoing error rate equivalent to the errorcount value stored in the outgoing line error register over a 100 msinterval.

Similarly, if the contents of the outgoing line error register 208 aregreater than or equal to 126, block 210 directs the microcontroller 130to use the contents of the outgoing line error register 208 to locate acorresponding index value in the forward index look-up table 140. Block210 then directs the microcontroller to use the located index value tolook up a corresponding hardware configuration record in the hardwareconfiguration look-up table 144, to look up register configurations ofthe TROHP 106 that must be implemented in order to invert bits of the B2line parity error bytes of the transport overhead portion 104 of theoutgoing communication signal 74, to effectively simulate a bit errorrate over a 100 ms interval equivalent to the bit error ratecorresponding to the index value that corresponds to the contents of theoutgoing line error register 208 (see Table 1 above). Themicrocontroller is then directed to set such register contents of theTROHP in accordance with the located record.

Block 210 continues to direct the processor circuit 102 to modifytransport overhead bytes of the outgoing communication signal 74 in theabove manner indefinitely, executed as a thread at the microcontroller130.

Thus, referring back to FIG. 3, it will be appreciated that as a resultof the execution of the transport overhead modification thread, a clearchannel has effectively been provided across a segment of the networkextending from the first network element 64 to the second networkelement 66.

For example, if new payload or path errors are being generated on theoptical pipe 72, or more generally, in any stretch of the customer'sequipment lying between the customer's last section- or line-terminatingequipment prior to the first network element 64, then without the clearchannel provided by the present embodiment, the customer would not havebeen able to detect such errors at the second customer equipment 98 forexample, as the relevant B1 and B2 section and line overhead bytes wouldhave been erased and regenerated by various STEs and LTEs on the network60, including the first and second network elements 64 and 66 andvarious elements therebetween. The customer would not have been able todetect such errors until arrival of the communication signal 74 at thecustomer's path-terminating equipment, at which point the customer wouldnot have any information as to the location along the path where sucherrors occurred.

In accordance with the present embodiment of the invention, however, theB1 and B2 bytes of the outgoing communication signal 74 transmitted fromthe second network element 66 have been modified, to simulate, for errorpreservation purposes, the effects that would have occurred if theoriginal transport overhead contents 82 had been transparently passedthrough the network 60 and had been adjusted to compensate for any patherrors that may have occurred on the network 60, to further enhance suchtransparency. For example, if path errors were occurring at an averagerate of 40 errors per 100 ms over the network 60, the B2 bytes and B1bytes of the outgoing communication signal 74 transmitted from thesecond network element 66, as a result of the addition of the contentsof the sum of differences register 203 to the reconstructed previoussection and line overhead fields 205 and 206 as discussed above, areeffectively adjusted to compensate for the new path errors: the new patherrors produce new parity errors, but the B1 and B2 bytes are alsoincremented to account for the new parity errors, so that any downstreamequipment counting the number of parity errors will not, on average,detect any discrepancy between the counted number of parity errors andthe expected number (B1 or B2) of parity errors. Accordingly, these patherrors on the network 60 do not, on average, result in any section orline error alarms at the second customer equipment 98 (although patherrors would ultimately be detected at the path terminating equipment).

However, if in addition to these path errors occurring on the network60, an average of 80 path or payload errors per 100 ms were occurring onthe optical pipe 72 prior to arrival at the first network element 64,these errors would not be reflected in the sum of differences valuesadded to the reconstructed previous section and line overhead fields 205and 206. Therefore, by adjusting the outgoing B1 and B2 bytes producedat the second network element 66 in response to the previous section andline overhead fields 205 and 206, the outgoing B1 and B2 bytes mimic theeffect of the original B1 and B2 bytes received at the first networkelement 64, so that on average, the values of the outgoing B1 and B2bytes will differ from the actual counted number of section and lineparity errors, by a rate equivalent to the rate at which the errors areoccurring on the optical pipe 72. Accordingly, when the outgoing signal74 produced by the second network element 66 arrives at the secondcustomer equipment 98, these errors trigger section error alarms and/orline error alarms, depending on whether the equipment 98 is both an STEand an LTE, indicative of an average parity bit error rate ofapproximately 80 errors per 100 ms.

Therefore, from the customers point of view, in terms of the customersability to detect section and line errors on the customers equipment, itis as if the network 60 and all the various LTEs and STEs therein didnot exist, and the optical pipe 72 was connected directly to the opticalpipe 96 shown in FIG. 3.

Alternatives

Although in the foregoing embodiment, the PPE portion 84 was describedas having been inserted into and extracted out of the communicationsignal 74 by hardware (the SYDR devices 90 and 108), alternatively, asimilar variation of TCM may be simulated with appropriate software ifdesired. Similarly, although the PTOH portion 80 was described as havingbeen inserted into and extracted from the communication signal 74 by theexecution of software, alternatively, if suitable hardware is available,the insertion and extraction of the PTOH portion may be carried out bysuch hardware. Depending on the configurations of such hardware,variations in the nature of the PTOH portion to more closely approximateor equal the original transport overhead contents 82 (B1 and B2) may beimplemented.

Likewise, although the modification of the transport overhead portion104 of the outgoing communication signal 74 in response to the PTOH andPPE portions was described as having been implemented through theexecution of software in combination with hardware, alternatively, suchmodification may be accomplished through hardware alone. It will beappreciated that such hardware, in combination with hardware forextracting the PTOH portion 80, may be capable of more refinedmodifications of the outgoing B1 and B2 bytes of the transport overheadportion 104, involving, for example, a more selective modification ofindividual B1 and B2 bytes, resulting in further enhanced transparency,with reduced or eliminated frame delay.

In addition, it will be appreciated from the foregoing that the methodsdescribed above for calculating, inserting and extracting the PTOHportion 80 are merely one example of simulating or approximating theeffects of transparent passage through the network 80 of the original B1and B2 bytes of the incoming transport overhead contents 82.Alternatively, other suitable simulation, approximation or transparencymethods may be substituted.

More generally, while specific embodiments of the invention have beendescribed and illustrated, such embodiments should be consideredillustrative of the invention only and not as limiting the invention asconstrued in accordance with the accompanying claims.

1. A method of providing clear channel access on a network, the methodcomprising: a) receiving a communication signal from a remote networkelement, said communication signal comprising a previous transportoverhead (PTOH) portion indicative of transport overhead contents ofsaid communication signal prior to arrival at said remote networkelement, and a previous path error (PPE) portion indicative of patherrors present in said communication signal at said remote networkelement; and b) modifying a transport overhead portion of saidcommunication signal in response to said PTOH and PPE portions.
 2. Themethod of claim 1 wherein modifying comprises calculating a differencebetween path errors present in said communication signal, and said patherrors present in said communication signal at said remote networkelement.
 3. The method of claim 1 wherein said communication signalcomprises a plurality of component signals and wherein modifyingcomprises calculating, for each of said component signals, a differencebetween path errors present in said component signal and path errorspresent in said component signal at said remote network element.
 4. Themethod of claim 3 wherein modifying further comprises calculating a sumof said differences of each of said component signals.
 5. The method ofclaim 4 wherein modifying further comprises adding said sum of saiddifferences to at least some contents of said PTOH portion.
 6. Themethod of claim 4 wherein modifying further comprises adjusting saidtransport overhead portion in response to a sum of at least somecontents of said PTOH portion plus said sum of said differences.
 7. Anapparatus for providing clear channel access on a network, the apparatuscomprising: a) a receiver operable to receive a communication signalfrom a remote network element, said communication signal comprising aprevious transport overhead (PTOH) portion indicative of transportoverhead contents of said communication signal prior to arrival at saidremote network element, and a previous path error (PPE) portionindicative of path errors present in said communication signal at saidremote network element; and b) a processor circuit in communication withsaid receiver and configured to modify a transport overhead portion ofsaid communication signal in response to said PTOH and PPE portions. 8.The apparatus of claim 7 wherein said processor circuit is configured tocalculate a difference between path errors present in said communicationsignal, and said path errors present in said communication signal atsaid remote network element.
 9. The apparatus of claim 7 wherein saidcommunication signal comprises a plurality of component signals, andwherein said processor circuit is configured to calculate, for each ofsaid component signals, a difference between path errors present in saidcomponent signal and path errors present in said component signal atsaid remote network element.
 10. The apparatus of claim 9 wherein saidprocessor circuit is configured to calculate a sum of said differencesof each of said component signals.
 11. The apparatus of claim 10 whereinsaid processor circuit is configured to add said sum of said differencesto at least some contents of said PTOH portion.
 12. The apparatus ofclaim 10 wherein said processor circuit is configured to adjust saidtransport overhead portion in response to a sum of at least somecontents of said PTOH portion plus said sum of said differences.
 13. Anapparatus for providing clear channel access on a network, the apparatuscomprising: a) means for receiving a communication signal from a remotenetwork element, said communication signal comprising a previoustransport overhead (PTOH) portion indicative of transport overheadcontents of said communication signal prior to arrival at said remotenetwork element, and a previous path error (PPE) portion indicative ofpath errors present in said communication signal at said remote networkelement; and b) means for modifying a transport overhead portion of saidcommunication signal in response to said PTOH and PPE portions.
 14. Acomputer-readable medium for providing codes for directing a processorcircuit to modify a transport overhead portion of a communication signalreceived from a network element, in response to a previous transportoverhead (PTOH) portion of said communication signal indicative oftransport overhead contents of said communication signal prior toarrival at said remote network element, and a previous path error (PPE)portion of said communication signal indicative of path errors presentin said communication signal at said remote network element.
 15. Asignal embodied in a carrier wave, the signal comprising code segmentsfor directing a processor circuit to modify a transport overhead portionof a communication signal received from a network element, in responseto a previous transport overhead (PTOH) portion of said communicationsignal indicative of transport overhead contents of said communicationsignal prior to arrival at said remote network element, and a previouspath error (PPE) portion of said communication signal indicative of patherrors present in said communication signal at said remote networkelement.
 16. A method of providing clear channel access on a network,the method comprising: a) inserting into a communication signal receivedat a network element, a previous transport overhead (PTOH) portionindicative of transport overhead contents of said communication signalprior to arrival at said network element, and a previous path error(PPE) portion indicative of path errors present in said communicationsignal at said network element; and b) transmitting said communicationsignal to a remote device.
 17. The method of claim 16 wherein insertingsaid PPE portion comprises calculating said PPE portion in response topath parity errors present in said communication signal at said networkelement.
 18. The method of claim 16 wherein inserting said PPE portioncomprises inserting said PPE portion into a path overhead portion ofsaid communication signal.
 19. The method of claim 18 wherein insertingsaid PPE portion further comprises adjusting a path parity error portionof said communication signal to compensate for insertion of said PPEportion into said path overhead portion.
 20. The method of claim 18wherein inserting said PPE portion comprises inserting said PPE portioninto an unused portion of said path overhead portion.
 21. The method ofclaim 18 wherein inserting said PPE portion comprises inserting said PPEportion into at least one of a Z3 and a Z4 byte of a Synchronous OpticalNETwork (SONET) path overhead portion.
 22. The method of claim 16wherein inserting said PTOH portion comprises inserting said PTOHportion into a transport overhead portion of said communication signal.23. The method of claim 22 wherein inserting said PTOH portion comprisesinserting said PTOH portion into an unused portion of said transportoverhead portion.
 24. The method of claim 22 wherein inserting said PTOHportion comprises inserting said PTOH portion into an unused time-slotof said transport overhead portion.
 25. The method of claim 16 furthercomprising calculating said PTOH portion in response to said transportoverhead contents of said communication signal prior to arrival at saidnetwork element.
 26. The method of claim 25 wherein calculatingcomprises counting a number of transport parity errors present in saidcommunication signal prior to arrival at said network element.
 27. Themethod of claim 26 wherein calculating comprises setting said PTOHportion equal to said counted number of transport parity errors presentin said communication signal prior to arrival at said network element.28. The method of claim 26 wherein calculating comprises setting saidPTOH portion equal to an index value indicative of said counted numberof transport parity errors present in said communication signal prior toarrival at said network element.
 29. The method of claim 16 furthercomprising inserting a PTOH header value in said communication signalpreceding said PTOH portion, to identify said PTOH portion.
 30. Themethod of claim 16 further comprising: a) receiving said communicationsignal at said remote device, said communication signal comprising saidPTOH portion and said PPE portion; and b) modifying a transport overheadportion of said communication signal in response to said PTOH and PPEportions.
 31. An apparatus for providing clear channel access on anetwork, the apparatus comprising: a) a processor circuit configured toinsert into a communication signal received at a network element, aprevious transport overhead (PTOH) portion indicative of transportoverhead contents of said communication signal prior to arrival at saidnetwork element, and a previous path error (PPE) portion indicative ofpath errors present in said communication signal at said networkelement; and b) a transmitter in communication with said processorcircuit and operable to transmit said communication signal to a remotedevice.
 32. The apparatus of claim 31 wherein said processor circuit isconfigured to calculate said PPE portion in response to path parityerrors present in said communication signal at said network element. 33.The apparatus of claim 31 wherein said processor circuit is configuredto insert said PPE portion into a path overhead portion of saidcommunication signal.
 34. The apparatus of claim 33 wherein saidprocessor circuit is configured to adjust a path parity error portion ofsaid communication signal to compensate for insertion of said PPEportion into said path overhead portion.
 35. The apparatus of claim 33wherein said processor circuit is configured to insert said PPE portioninto an unused portion of said path overhead portion.
 36. The apparatusof claim 33 wherein said processor circuit is configured to insert saidPPE portion into at least one of a Z3 and a Z4 byte of a SynchronousOptical NETwork (SONET) path overhead portion.
 37. The apparatus ofclaim 31 wherein said processor circuit is configured to insert saidPTOH portion into a transport overhead portion of said communicationsignal.
 38. The apparatus of claim 37 wherein said processor circuit isconfigured to insert said PTOH portion into an unused portion of saidtransport overhead portion.
 39. The apparatus of claim 37 wherein saidprocessor circuit is configured to insert said PTOH portion into anunused time-slot of said transport overhead portion.
 40. The apparatusof claim 31 wherein said processor circuit is configured to calculatesaid PTOH portion in response to said transport overhead contents ofsaid communication signal prior to arrival at said network element. 41.The apparatus of claim 40 wherein said processor circuit is configuredto count a number of transport parity errors present in saidcommunication signal prior to arrival at said network element.
 42. Theapparatus of claim 41 wherein said processor circuit is configured toset said PTOH portion equal to said counted number of transport parityerrors present in said communication signal prior to arrival at saidnetwork element.
 43. The apparatus of claim 41 wherein said processorcircuit is configured to set said PTOH portion equal to an index valueindicative of said counted number of transport parity errors present insaid communication signal prior to arrival at said network element. 44.The apparatus of claim 31 wherein said processor circuit is configuredto insert a PTOH header value in said communication signal precedingsaid PTOH portion, to identify said PTOH portion.
 45. A systemcomprising the apparatus of claim 31 and further comprising said remotedevice, said remote device comprising: a) a receiver operable to receivesaid communication signal comprising said PTOH portion and said PPEportion; and b) a processor circuit in communication with said receiverand configured to modify a transport overhead portion of saidcommunication signal in response to said PTOH and PPE portions.
 46. Anapparatus for providing clear channel access on a network, the apparatuscomprising: a) means for inserting into a communication signal receivedat a network element, a previous transport overhead (PTOH) portionindicative of transport overhead contents of said communication signalprior to arrival at said network element, and a previous path error(PPE) portion indicative of path errors present in said communicationsignal at said network element; and b) means for transmitting saidcommunication signal to a remote device.
 47. A computer-readable mediumfor providing codes for directing a processor circuit to: a) insert intoa communication signal received at a network element, a previoustransport overhead (PTOH) portion indicative of transport overheadcontents of said communication signal prior to arrival at said networkelement, and a previous path error (PPE) portion indicative of patherrors present in said communication signal at said network element; andb) transmit said communication signal to a remote device.
 48. A signalembodied in a carrier wave, the signal comprising code segments fordirecting a processor circuit to: a) insert into a communication signalreceived at a network element, a previous transport overhead (PTOH)portion indicative of transport overhead contents of said communicationsignal prior to arrival at said network element, and a previous patherror (PPE) portion indicative of path errors present in saidcommunication signal at said network element; and b) transmit saidcommunication signal to a remote device.